The linear, single-stranded DNA from the parvovirus KRV contains regions of double strandedness. These double-stranded regions contain 230 to 300 nucleotides and involve the 5' and probably the 3' termini. They may be involved in viral DNA replication. A double-stranded form of KRV, DNA, probably an intermediate in DNA replication, is synthesized by the infected cell 4 to 6 hours after infection. The synthesis of double-stranded DNA decreases as virion single-stranded DNA synthesis increases. A nucleoprotein complex has been isolated from KRV virions. The complex contains KRV DNA and a protein with the requirements, activity and size of the KRV-associated DNA polymerase. We have found that rat adenocarcinoma cells 13762 (a DMBA induced tumor) and another rat tumor line (C58NT)D are inhibitory to lymphocyte proliferative responses. This inhibition is due to a contamination of the line with KRV. The presence of such viruses in in vivo passaged tumor lines may adversely affect the measurement of lymphoproliferative responses to tumors in vitro.